1. Field of the Invention
The invention is directed to a solar energy collection and storage apparatus including a storage container having a transparent covering and a dark absorbent bottom, as well as supply and offtake lines for fresh and household or industrial water.
2. The Prior Art
The discontinuous availability of sunlight makes storage equipment or reservoirs indispensable for autonomous systems or those that use the energy optimally. This is also true for the field of solar hot water production.
Especially in the period since 1973, variants of such solar hot water systems have been developed in virtually all the industrially developed countries. In principle, these systems can be classified as either large-surface-area systems with reservoirs of high volume and long storage time, or smaller systems with smaller reservoirs and a shorter storage time.
For economic reasons, variants in both classifications have recently appeared that feature a compact unit combining a solar energy collector and reservoir. In the field of the large systems, what is known as solar ponds have been developed, while in the field of smaller systems, combined collection and storage collectors, known as internal storage collectors, have been developed.
Solar ponds are based on a physical effect, discovered about 1900 in Hungary and technically improved in the 1950s in Israel (particularly by Tabor, et al.), according to which the surface water in a pond several meters deep is fresh water, while the layers below it are salt water, which increases in concentration toward the bottom. The sunlight penetrating to the dark bottom is absorbed there and converted into heat, but cannot--unlike the situation in typical freshwater ponds--be transported by convection to the surface, because the layers of water at the bottom are heavier. In this way, the lower portion of the solar ponds heats up and is relatively well insulated thermally by the fresh water layers located above it. The useful heat is then drawn from the lower portion of the solar pond, using a heat exchanger.
In practice, such solar ponds are used for instance to drive thermodynamic machines with drive fluids having a low boiling point; because of their function of solar energy storage around the clock, they can be used to produce mechanical energy or electric current even in periods of bad weather, if the solar pond is suitably dimensioned.
The internal storage collectors, in the simplest case, comprise black water-filled containers located inside an outer container that provides good insulation, the surface which is facing the sun may be provided with one or more glazed windows. The water that heats up in the interior of the black container may be delivered to the consumer either via heat exchangers or directly. An arrangement of this kind is known for instance from German Pat. No. 26 39 425.
Although the basic simplicity in design of the arrangements described greatly facilitates their actual manufacture, nevertheless they have very serious disadvantages, in particular because they still have a very low efficiency.
The substantial disadvantage of solar ponds is that at typical depths of three meters, the useful energy available at the bottom of the solar pond is only about 35%, because of the extinction of the longer-wave portion in the solar spectrum; this directly limits the maximum possible efficiency. Moreover, the stratification of such solar ponds with layers of salt water counterracts natural convection and must therefore be continuously maintained by pump circulation in order to keep the gradient constant. Finally, the free surface of the water is very vulnerable to the wind, especially when the surface area is large, as is desired, because the waves produced by the wind disrupt the stratification of the water layers and cause losses.
The substantial disadvantage of internal storage collectors is the great thermal inertia of the volume of water to be heated. Precisely in climatic regions having a fluctuating amount of sunshine, collectors of relatively small thermal capacity are advantageous, because they already furnish the required useful temperatures when the periods of sunshine are short. Storage collectors of the classic type are therefore limited in their use to countries that have a great deal of sunshine, and even then can be used only for applications in which hot water is not needed until the second half of the day.
This basic shortcoming of the system can be partly overcome, for example by isothermal heating in a storage collector of variable volume, such as that described in European patent application Ser. No. 0 219 566.
In this previously known arrangement, solar energy is absorbed continuously through an absorber hose that is transparent at the top and dark at the bottom; the fill level of the storage collector varies as function of the desired set temperature and the intensity of the incident sunshine. The system is now no longer thermally sluggish, and it operates at high efficiency. However, the water in the collector hose is not under pressure and must be brought to useful pressure with a supplementary pump. The heat losses of the reservoir toward the top are limited by a transparent insulation, but the the system still does not attain the insulating properties of nontransparent insulating materials such as polyurethane foam.